Page 6 - 4017_terraces_memdes_lviii_95_100

SUBMERGED DEPOSITIONAL TERRACES ALONG THE ITALIAN COAST - CONCLUSION 115

(angles of foreset, reactivation surface) that emerge from the comparison between the sismoacoustic
profiles of the SDT and the examples in emergence of slopes with a medium-high energy (see Crotone
basin, MASSARI et alii, this volume).

An alternative interpretation, based on considerations expressed in CHIOCCI & ORLANDO, 1966 and
in CHIOCCI & ROMAGNOLI (this volume), considers the SDT as depositional wedge of enterely subma-
rine formation, sedimented below the wave base level due to mobilization and redistribution of the
sediments produced by the littoral erosion in response to meteo-marine events of greater energy
(wave-formed terraces). According to this second interpretation, the SDT found at a shallow depth
(like those observed around the Aeolian and Pontine islands, with depositional edge at about 20-35 m),
might represent very young depositional bodies, connected to the actual sea level highstand (develo-
ped in the last 6 ka) and, therefore, referable to present-day depositional processes. To sustain this
hypothesis, the followings can be considered: (1) the lack of physical continuity of shallow (actual)
SDT with littoral deposits that, in the facing coasts assume the form of rare and mosly gravelly poc-
ket beach of limited extension ; (2) the high steepness of the foreset, that seems to approximate the
angle of rest of the materials, i.e. the effect of avalanching on a submarine slope; (3) the direct obser-
vation of bedforms down to more than -30 m on the upper surface of a SDT of the Aeolian Archi-
pelago, that evidence the actual action of the wave motion until the terrace edge during storms.

APPLICATIONS 1: NEOTECTONICS

One of the classical application field of the study of coastal marine terraces (see SPOSATO, this
volume) is the definition of the vertical mobility of the continental margin. At present, the height of
the Thyrrhennian deposits is still the most valuable indicator of the neotectonic trend of a coast for
the the last 125 ka, that is to say for the last interglacial/glacial/interglacial cycle.

Such an application is possible also for the submerged depositional terraces, despite the morpho-
logic, stratigraphic and genetic differences between these and the coastal terraces. In fact, also the
SDT are controlled by the (paleo)level of the sea, as it is shown by the fact that they are found on the
sea floors at a quite costant depth even for tens of kilometers.

There are some limitations to their use: a) the relation between the depth of the main depositional
parameters of SDT and the paleolevel of the sea is not completely clear and, in the case of the hypo-
thesis of a genesis connected to the wave base level , factors such as fetch and wave refraction might
assume a relevant importance (this uncertainty, that is a common problem to the coastal terraces, does
not prevent a comparative use of the SDT in a given area); b) the fact that the SDTs are not always
present (they are present only on very steep edges) and, where present, are generally located in the
external platform at a certain distance from the coast, where it is common to have also loading subsi-
dence effects; c) the usual lack of dating due to the difficulty of sampling, at least with cheap tech-
nologies, of the deposits lying on the sea floor.

Despite these limitations, some indications of neotectonic have been taken from some cases stu-
died in this Atlas. So, the gradual variation of depth (2m/km) of the SDT in the western Pontine Isles
was congruent with the entity of the uplifting estimated for an Holocene beach in Palmarola Isle; the
lack of observation of different kind of terraces on the continental platform of Calabria is congruent
with the regional tectonic uplift that might have caused the emersion and the removal of all the older
SDT; finally the presence, in areas characterized by actual and/or recent volcanism, of sectors with
different or alternated vertical mobility, opposed to the relatively constant depth of the SDT in rela-
tive stabile continental edges is a convincing data.

APPLICATIONS 2: COMPARISON WITH SIMILAR FEATURES PRESENT WITHIN
CONTINENTAL MARGINS

The genesis of TDSs and their age (where sampled), indicates these forms as produced by deposi-
tional processes at or near the shoreface during last sea level lowstand (Würm glacial maximum, 20,000
years b.p.).

The published eustatic curve , based on oxygen isotopes, indicate sea level lowstands roughly every
100,000 years. Such lowstands cause a renewal of the depositional systems because of sea-level fall,